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;                                        Various Medium-speed Engines.                631

to the latter.     The practical objections were  the difficulty  of
packing the necessarily large glands, and of getting at the trunk -
pin;  but a more serious objection was  the increased  cooling
I                   surface.     Maudslay's engine was essentially the steeple  engine

;                   laid horizontally, the air pump being worked from a projection

on one of the piston rods.    The packing of the parallel glands
was the only difficulty.

The modern marine engine is always either compound, triple,

or quadruple in design, the two-cylinder compound being shewn

at L, which also serves to explain the triple or quadruple.    The

type is  known  as  the 'vertical inverted/  or 'steam hammer/

and is merely a direct-acting vertical engine with cylinder above

i                   and crank below, to give sufficient propeller immersion with direct

driving.    The slide valves are driven by eccentrics as at j, and

the air pump by a rocking lever.    The surface condenser is cast

with the standards, on one side, and the exhaust steam sometimes

passes through one of the standards;   but the method  is  not

advised by some engineers, because of irregular alignment caused

:                    by expansion.    When the triple engine is adopted, the valves are '-

either placed between the cylinders, or as at R, on one side.    In

the latter case the valve gear must be somewhat altered,    a, l>,

I                    and c are the cylinders seen in plan, and dy e, f the respective

;                    valves:  in this example  of piston form.     The passage of the

>                    steam will be understood from the sketch, entering first through

t                   d to #, then through e to , through /to c, and finally out to the

condenser.

High-speed Engines are a class of engine, usually of
small proportions, making 500 revolutions per minute or more.
A few principal examples are given at Fig. 627. A and B are
types of the rotary engine, much in favour with inventors about
the year 1870 and previously, but now practically discarded. A may-
be called the f annular' and B the ' eccentric' type, a sliding
* abutment' a being required in each case to receive the re-
actionary pressure. There were difficulties in these engines
regarding packing and expansive working. Willans' side-by-side
three-cylinder engine c, and Brotherhood's three-cylinder engine D,
dispense with valve gear. At c the piston rods a, b, c, act as
valves, each admitting or cutting off steam to the next high-